1. Field of the Invention
This invention relates to soldering of electronic components and in particular to soldering involving solder fluxes and solder pastes.
2. Art Background
For many applications electronic components are surface mounted on a printed wiring board (PWB). This surface mounting is accomplished by positioning a component on a metallic contacting region of a PWB and introducing a solder paste between the leads and the corresponding contact pads. The assembly is then heated to a reflow temperature at which the soldering material flows and an electrically conductive joint between the lead and the solder contact pad is formed.
The solder paste in this soldering process contains solder balls of relatively small dimension, i.e., balls having a diameter in the range 5 .mu.m to 100 .mu.m, and a vehicle i.e., flux. The solder balls are used to ensure the presence of sufficient electrical conductor to produce a low resistance joint between the component lead and the PWB contacting pad. The flux is employed to yield a variety of properties necessary for the soldering process. In particular, the flux is chosen so that it is possible to print the paste onto the contacting pads of the PWB through a stencil without depositing essentially any paste in other regions of the PWB. The paste is also chosen 1) to have good tackiness so that when the component lead is pressed onto the paste, it remains in position and 2) so that at the reflow temperature any solder in the paste that is not precisely aligned with the contacting pads moves into the desired pad region through surface interactions. The flux also contains a dissolving agent that removes any oxide coating from the solder balls, the contacting pads and from the component leads.
In addition to all the other requirements placed on the solder flux, it is essential that after reflow any residue which remains is either non corrosive and cosmetically appealing or is removable by cleaning. Very few fluxes are available that leave no undesirable residue. Most fluxes leave residues which are cleaned with organic solvents. Environmental considerations presently have imposed strong impetus to formulate fluxes so that residue is removable with an aqueous medium.
Although aqueous cleanable residues are acceptable, the additional step of cleaning represents a concomitant additional cost. Therefore, although not essential, it is certainly desirable to formulate a solder flux that leaves no objectionable residues even without a cleaning procedure. A solder flux that satisfies all the required properties for the vehicle and that also produces no unacceptable residue is extremely difficult to formulate. This difficulty in formulation is especially severe, since any residue remaining must not hinder subsequent electrical testing of componends by applying a probe to the solder connection.